Looking a bit further, I see in the generated build-outputs.mk a rule to
make mod_dav_svn.la that uses $(LINK). $(LINK) includes "-rpath $(libdir)".
So this command must be changed to allow it be installed to the correct dir.

I changed ac-helpers/svn-apache.m4 and Makefile.in to set APACHE_LIBEXECDIR
to be this directory, via apxs. But I'm not sure how to plug it in once I've
got it. I manually added this line to build-outputs.m4:

subversion/mod_dav_svn/mod_dav_svn.la: libdir=$(APACHE_LIBEXECDIR)

and that seemed to work. But how do I duplicate that with changes to
build.conf and build/gen_base.py instead?